The present invention relates to a metallic foil for a metallic honeycomb used as a catalyst support, or the like, for purifying the exhaust gas of an automobile, a motor-cycle or the like, and to a catalyst support for exhaust gas purification made by using the metallic foil.
In recent years, with the tightening of automobile exhaust gas regulations, metallic honeycombs used in place of conventional ceramic honeycombs form a gradually increasing proportion of the honeycombs used as catalyst supports for exhaust gas purification. This is because a metallic honeycomb having a wall thickness thinner than that of a ceramic honeycomb is excellent in early-stage purification capacity since it quickly rises in temperature when an engine starts from a cold state and has further advantages such as low exhaust resistance.
As for the structure of a metallic honeycomb, a metallic foil of Fexe2x80x94Crxe2x80x94Al steel having a thickness of 15 to 50 xcexcm is corrugated into a sinusoidal wave shape, a triangular wave shape, a trapezoidal wave shape or the like, and the corrugated sheet is laid on top of a flat sheet, or on top of another corrugated sheet, to form a rectangular parallelepiped-shaped structure, or these are wound together to form a cylindrical structure, a racetrack-shaped structure or the like, and any of these is used as the structure of a metallic honeycomb. This metallic honeycomb is inserted into a jacket made of heat-resistant stainless steel to form a catalyst support (metallic support) for exhaust gas purification. Also, a metallic honeycomb, wherein the corrugated sheet and the flat sheet, or the corrugated sheet and the other corrugated sheet, are joined to each other at all or some of their overlapping portions is used.
However, a metallic honeycomb thus formed by using a sinusoidal wave-shaped corrugated sheet forms acute-angled open spaces 4 in the proximity of a contact point of the sinusoidal wave-shaped corrugated sheet 2 with a flat sheet 1 as shown in FIG. 1 and causes a catalytic substance 3 composed of xcex3-alumina, precious metal catalyst particles, a co-catalyst, etc. to penetrate deep into the spaces. The portions of the catalytic substance having penetrated into the spaces do not contribute in substance to a catalytic reaction since they are 20 xcexcm or more distant from the surfaces of the catalytic substance directly touching the flow of engine exhaust gas, which is a shortcoming causing a waste of the expensive precious metal catalyst.
For this reason, a trapezoidal corrugated sheet 2 has recently been required wherein the trapezoidal shape has corner portions bent more sharply, in corrugation work of a foil, as shown in FIG. 2. This is because the use of a trapezoidal corrugated sheet can diminish the spaces formed in the proximity of a contact point of the trapezoidal corrugated sheet 2 with a flat sheet 1 and thus reduces the amount of useless portions of a catalyst not contributing to a catalytic reaction.
As for a manufacturing method of a trapezoidal corrugated sheet, it can be manufactured by rotating a pair of toothed rolls in mesh and passing a flat foil between the toothed rolls, as disclosed in Japanese Unexamined Patent Publication No. H8-257412, for example.
However, when a conventional metallic foil undergoes the corrugation work of bending the corner portions of a trapezoidal shape more sharply, a problem arises that cracks or ruptures frequently develop during the work and thus the productivity is considerably lowered.
The object of the present invention is, in view of the above problem, to provide a metallic foil, to be used for a metallic honeycomb for purifying the exhaust gas of an automobile or the like, not causing cracks or ruptures to develop in the foil even when the metallic foil undergoes corrugation work of bending the corner portions of a trapezoidal shape more sharply, and to provide a catalyst support for purifying exhaust gas of an automobile or the like made by using the metallic foil.
The present inventors studied the cause of cracks or ruptures developing in a metallic foil during corrugation work and have found that the prevention of cracks or ruptures developing in a metallic foil during corrugation work can be achieved by adjusting the surface roughness of the metallic foil. The present invention has been completed on the basis of the finding and the gist of the present invention is as follows:
(1) A rough surface finished metallic foil with good corrugation workability, which metallic foil is made of heat-resistant stainless steel and is used for a catalyst support for exhaust gas purification, characterized by having surface roughness in an as rolled or after being polished state of not less than 3.2 xcexcm and not more than 5.0 xcexcm in Rmax.
(2) A catalyst support for exhaust gas purification having a structure configured by inserting a honeycomb formed of a metallic foil made of heat-resistant stainless steel into a jacket made of heat-resistant stainless steel, characterized in that said metallic foil is a rough surface finished metallic foil having surface roughness in an as rolled or after being polished state of not less than 3.2 xcexcm and not more than 5.0 xcexcm in Rmax.